Tiling over concrete cracks

Q: The Florida home I just purchased has many cracks in the concrete slab, some of them 10 ft. long and up to 1/8 in. wide. I’d like to install a tile floor over the slab, but I’m worried that the tile will crack. Do you have any advice?

Bill Diaz, San Juan, PR

A: Tom Meehan, a tile installer and owner of Cape Cod Tileworks in Harwich, Massachusetts, replies: Cracks in a concrete slab are a problem that I run into frequently. In some cases I’ve seen cracks that are almost 1/4 in. wide, so 1/8-in. cracks are nothing out of the ordinary. Applying tile in these situations is really quite simple.

Before I begin the process of tiling over a cracked slab, I prep the slab with a crack-isolation membrane. A product that I’ve had good luck with is NobleSeal CIS (The Noble Co.; www.noblecompany.com; 800- 878-5788), a composite sheet membrane that is made from chlorinated polyethylene with woven polyester fabric laminated to both sides. The membrane, which is bonded to the slab with the same latex-modified thinset that I use for installing the tile, absorbs any additional movement in the concrete. And at only 0.8-mm thick, the membrane can be used under almost any tile floor.

I cut the membrane to extend at least two tile widths on both sides of the crack. Next I spread thinset over the slab where I’ll be applying the membrane. After the sheet membrane is laid down over the thinset, I use a 75-lb. roller to flatten it and to squeeze out any air pockets. If I encounter a slab with a lot of cracks, I apply membrane over the entire floor. It’s easy to put down, and it may save me time and money in the long run.

Although I’ve had tremendous luck with this product and this method over the years, I always let my customers know that it is possible that this fix might not work. Cracks in some slabs may not be controllable because of the soils that are under the slab, and there is a chance that the slab was poured incorrectly. If I have any doubts about the strength or stability of the slab, I don’t proceed with the project without an engineer’s approval.